In recent years, metal implants have increasingly been used widely in the fields of orthopedics and dentistry, such as artificial bones and artificial tooth roots. For example, when the function of a joint has been lost due to arthrosis deformans or rheumatoid arthritis, medical treatment for regaining the function by exchange to an artificial joint has become general.
As the method for fixing artificial joints to bones, two main types of methods are presently used. One is a technique of filling an adhesive called bone cement into a gap between a bone and an artificial joint to fix them. Since bone cement hardens during the operation, it becomes possible to start rehabilitation early after the operation. However, its use tends to decrease year by year because the risk of causing a shock disease or a blood pressure decline due to excessive compression to the bone marrow during the filling of bone cement has been reported. Another method is a fixing method called cementless fixation, which uses no bone cement. One example is a method of fixing by a mechanical anchoring effect caused by intrusion of a surrounding bone into a porous part formed in the surface of an artificial joint. Since this method can avoid the risk caused by use of bone cement, the cases using the method are increasing rapidly. However, since the time needed for an artificial joint to be fixed to a bone depends on the rate of growth of patient's bone, the patient is required to take a long rest.
In order to shorten the resting period when the aforementioned cementless fixation is adopted, some methods for imparting osteoconductive property to artificial joints have heretofore been investigated. One of them is a method in which osteoconductive property is imparted to the surface of an artificial joint by spraying hydroxyapatite, which is a bone-like component, at high temperatures, and it has already been in practical use. It, however, is supposed that this method has problems that large-scaled equipment for spraying is required, that apatite to be sprayed may be degraded due to exposure to high temperature, and that an apatite layer formed may exfoliate.
Patent document 1 discloses a bone substitutive material provided with specifically sized ruggedness and an alkali titanate layer on a bonding surface to a body tissue, which is a surface of a base material made of titanium or titanium alloy. It is disclosed that the bone substitutive material exhibits improved apatite-forming ability by having an alkali titanate layer on a base material surface and that a strong fixing force to a living bone by an anchoring effect due to the ruggedness can be obtained. As examples of the method for forming the ruggedness, sandblast treatment, and a method of spraying powder are provided. The aforementioned alkali titanate layer is formed by forming a layer of hydrated gel of sodium titanate by immersing a base material in an aqueous sodium hydroxide solution, followed by calcination. Although it is conceivable that the surface layer formed by this method is composed of a metal oxide layer containing titanium and sodium, it is not easy to completely confirm safety of such novel materials to living bodies.
Patent document 2 discloses an osteoconductive biomaterial comprising a metal base material containing titanium and a metal oxide layer formed on a surface of the metal base material, wherein at least a surface of the metal oxide layer has a chemical species composed of TiOH. The osteoconductive biomaterial having such a chemical species on its surface is formed by hydrothermally treating, under conditions including a temperature of 100° C. or higher and a pressure of 0.1 MPa or higher, a titanium oxide layer obtained by thermally treating a metal base material containing titanium at a temperature of 1000° C. or lower. At this time, the preferable thickness of the metal oxide layer formed by the thermal treatment is about 3 to about 10 μm. By adopting such a constitution, it is possible to provide a biomaterial with good osteoconductive property. For example, in Example 1 of Patent document 2, it is disclosed that a sample which had been obtained by forming a metal oxide layer of about 5 μm in a thickness by thermally treating a Ti-29Nb-13Ta-4.6Zr alloy at 800° C. for 1 hour, immersing the resultant in a phosphate buffer, and hydrothermally treating it under conditions of 120° C. and 0.2 MPa generated apatite crystals in a simulated body fluid. On the other hand, Comparative Example 2 of Patent document 2 discloses that no apatite crystals can be formed by only forming a metal oxide layer without conducting the aforementioned hydrothermal treatment. Moreover, Patent document 2 has no particular description concerning the shape of the surface of a metallic base material.
Non-patent document 1 discloses the result of the observation of apatite forming conditions by immersing, in a simulated body fluid, a titanium metal flat plate sample on the surface of which an oxide film had been formed by heat treatment in the air at 400° C. for 1 hour. In the experiment, the container containing the simulated body fluid was a polystyrene container having an upwardly curved bottom surface and the flat plate sample was immersed therein in such a way that the sample was placed on the curved bottom. Then, no apatite was formed on the upper surface of the sample, but formation of apatite only on the under surface (the side which comes into contact with the bottom of a container) was observed. Since the under surface of the sample was in contact with the curved surface of the container, the gap depended on the location, but in general apatite was easily formed at places where there was a gap of about 100 μm. However, the reason why apatite is formed at such places is not described. Moreover, non-patent document 1 has no particular description concerning the shape of the surface of a metallic base material.
Patent document 1: JP-A 2000-210313
Patent document 2: JP-A 2003-235954
Non-patent document 1: Xiao-Xiang Wang et al., three others, “A comparative study of in vitro apatite deposition on heat-, H202-, and NaOH-treated titanium”, Journal of Biomedical Materials Research, John Wiley & Sons, Inc., Periodicals, Inc., 2001, vol. 54, pp. 172-178